There are thirty sentences in this section. Beneath each sentence there are four words or phrases marked A, B, C and D. Choose one word or phrase that best completes the sentence.
Mark your answers on your answer sheet.
It is essential that these application forms ______ back as early as possible.

A．to
B．for
C．of
D．on

A．Why it is difficult to use aerial photographs in research.
B．Why oceanic research is so limited.
C．How oceanic research has helped land research.
D．How light is used to attract sea lif